High surface aluminum oxide particles are well known in the art. Conventionally they are precipitated from either a liquid or a gas phase. In the usual liquid phase embodiments, the acidity, temperature or concentration of the aluminum salt solution is varied to precipitate the alumina oxide or hydroxide. Alternatively an organic aluminum or aluminum oxide complex, such as an alkoxide, is hydrated to precipitate the inorganic oxide or hydroxide. In the usual gaseous phase embodiment an aluminum compound is evaporated and then hydrolyzed by steam or oxygen to precipitate the metal oxide. Representative of that prior art method is the formation of fumed alumina from vaporized aluminum chloride. In both the usual liquid and gas phase precipitations the particles formed can be quite fine and have quite large specific surface areas; however, these powders do not possess an internal foam structure.
The decomposition or hydrolysis of metal salts in high temperature gas streams is common in the art. In the usual approach, known as the spray dryer technique, a slurry or solution of a decomposable salt is atomized into a gas. The particles so formed can be relatively large, hollow spheres. The spheres often contain holes where the residual water or decomposition gases have escaped from the interior of the particle. The hollow spheres formed under these conditions do not have high specific surface areas.
A method for high temperature hydrolysis of a metal salt solution is disclosed by Walsh in U.S. Pat. No. 3,273,962 and in an article entitled, "Ultrafine Metal Oxides By Decomposition of Salts in a Flame," Ultrafine Particles (Wiley & Sons, 1963). In this method, a two fluid atomizing nozzle is used to form a plume of solution droplets. This plume may contain, or is subsequently combined with, the fuel and oxidizing gas for a flame. Subsequent to creation of the droplets, a pilot flame initiates combustion. When combustion occurs, the fuel gases are burned to provide heat and thus permit decomposition of the metal salts. Walsh discloses a one-step method for making alumina particles from aluminum sulfate solutions. However, the resulting product is a plate like or laminar structure with a surface area of 33 M.sup.2 /gm. Walsh does not teach the formation of foamed alumina from aluminum sulfate solutions.
A Pennsylvania State University thesis by Renato Ciminelli entitled "Synthesis of Alumina from Al (NO.sub.3).sub.3 9H.sub.2 O by the Evaporative Decomposition of the Solution Process" discloses the decomposition of aluminum nitrate in a high temperature atmosphere. Ciminelli uses an atomizing nozzle to spray aluminum nitrate solution into a furnace heated ceramic tube. Ciminelli found that aluminum nitrate droplets converted to aluminum oxide particles, with a hollow sphere or foam structure, when the temperature of the outside of the ceramic tube, midway along its length, was in a range from 700.degree. C. to 1000.degree. C. Under these conditions, Ciminelli was unable to make alumina particles with surface area greater than 43 square meters per gram. Ciminelli also calcined the particles made in his apparatus. By further heat treating the alumina particles at 950.degree. C. he was subsequently able to raise their surface area to 77 square meters per gram.